#hypoxic zone

Excerpt from this Sierra Club story:

In recent years, their forecasts have been pretty good. But for 2019, there’s a chance the models may be off. “This year’s historic and sustained river flows will test the accuracy of these models in extreme conditions, which are likely to occur more frequently in the future according to the latest National Climate Assessment,” Steve Thur, director of NOAA’s National Centers for Coastal Ocean Science, said in a statement. “The assessment predicts an increase in the frequency of very heavy precipitation events in the Midwest, Great Plains, and Southeast regions, which would impact nutrient input to the northern Gulf of Mexico and the size of the hypoxic zone.”

The largest dead zone since the hypoxia was first measured in 1985 took place in 2017, when heavy spring rains and stream flows dumped lots of nutrients into the river basin, swelling the dead zone to 8,776 square miles. That was well above the five-year average of 5,770 square miles.

In the 1960's scientists discovered what they called zones of low oxygen or hypoxic dead zones in the ocean — usually nearly densely inhabited coastlines — where marine live could not be sustained. (1) According to an article in Science, these zones have doubled approximately every 10 years since the 1960's. (5)

Dead zones occur when burning fossil fuels, sewage and agricultural run-off (fertilizer) find their way into the ocean. These excess nutrients (especially phosphorus and nitrogen) acts to fertilize algae causing an excess of plant growth called algal blooms. (8)

The blooms photosynthesize during the day but at night deplete oxygen from the water column. (1) When the blooms decompose, they consume oxygen from the surrounding water, casing eutrophication, draining the water of all available oxygen where no living organisms can survive. (4) Naturally occurring coastal upwelling along continental margins can also lead high algal productivity. (5)

Dead zones are usually found at the mouths of bodies of water where pollution and fertilizers build up. (9) In 2003 the U.N. Environment Programme reported 146 dead zones in the world's oceans where life could no longer be sustained. A study in 2008 confirmed 405 zones along more than 95,000 square miles of sea, about half the size of California or the size of New Zealand. (9) 

One of the largest dead zones in the U.S. is located on the Mississippi in the Gulf of Mexico, spanning from Louisiana to Texas. The accumulation of nutrients here is greatly affecting the shrimping industry. (1) And as a result of the Deep Horizon oil spill, the dead zone is reported to be getting larger. According to the United Press International, "the 2010 dead zone could be between 6,500 and 7,800 square miles, an area roughly the size of Lake Ontario." (10)

A large low-oxygen zone in the Northwest U.S. was found 1,200 square miles along Oregon in 2006. (6) The largest dead zone is located 8,500 square miles along Baltic Sea where the water is actually hypoxic nearly all year long. (9)

Below is a world map of hypoxic zones in the world's oceans from Science. 

These dead zones contribute to climate change, destroy habitats, kills animals, and disrupt food webs. "Eutrophication-induced hypoxia" usually occurs once a year during the blooming seasons in spring and summer when the water is warmest. (5)

More algae and less oxygen causes sudden mass migrations of fish and molluscs and mass deaths of bottom dwellers or sessile animals who are unable to locate. (4) The increase is fertilizers in the oceans and low oxygen levels also lead to reproductive problems in fish, leading to low egg counts or lack if spawning. (1)

In addition to killing marine life and affecting marine food webs, this eutrophication also can heavily impact the fishing industry. In 1976, for example, low oxygen across 385 square miles from New York to New Jersey cost commercial fishing $500 million and in The Gulf of Mexico, more than 212,000 metric tons of seafood is lost to hypoxic waters. (4)

Hypoxic zones can also increase levels on nitrous oxide in the atmosphere, further exacerbating global warming by increasing the sizes of holes in the ozone and increasing our exposure to UV radiation. (4)

The good news is that some dead zones are reversible, although only a few have been. Dead zones can be diminished when fertilizer run-off is reduced. (4) An unintentional example was seen between 1991 and 2001 when the Black Sea saw a reversal period after fertilizer became too costly to use. Of course, this was due to the collapse of Soviet Union and not conservation efforts, but it was a great example and set precedent for dead zone reversal. (1) The U.N. has advocated cleanups by setting policy to reduce emissions. Another occurred from 1965 to 2000 in the North Sea where policy efforts reduced nitrogen emission by 37 percent. (4)

Unfortunately, climate caused dead zones nay not be so easy to reverse. Scientists are saying that the Pacific Ocean dead zone in North America may not be reversible. Evolving wind conditions brought on by climate change are thought to be responsible as are enlarged ozone holes. (4) If it were a result of pollution, it would be easier or possible to reverse. But with these conditions, scientists say that the dead zones will only increase. (2)